Anchor chains have been used for many years to moor vessels. The standard technique for hauling in, tensioning and holding the chain has been through the use of windlasses, generally having a 5 or 7-whelp pocket drive wheel designed as a wildcat. The windlass is powered by either electric motor, steam, or hydraulic motor via a shaft and gear mechanism. Brakes of the disc or band type are provided to act on the wildcat rim or the shaft. Band-type brakes are the most common. The mooring anchor chain is held by a passive stopper that holds the chain either directly on a link or engaged into the wildcat of the windlass. While the vessel is underway, the anchor, anchor chain and the associated equipment are stored on the vessel. A chafing device, referred to as a fairlead, is used as a guide to absorb the wear on the vessel as the mooring lines are deployed or hauled in when the location is reached.
In the 1950s, with the advent of the offshore petroleum industry, various vessels started to be used which required spread mooring systems to hold the vessel on station as the unit drilled or stored crude oil. These vessels have from 4-12 mooring lines consisting of anchor chain, wire rope, or a combination of the two.
These vessels are moved on an average of three or more times a year, but as often as 12-15 times. In view of the frequent movements, it is advantageous, when using anchor chain, that such vessels have an equal number of windlasses as there are mooring lines. These mooring systems require considerable space and are costly, as well as heavy. It is not unusual for the mooring systems on drilling rigs and production vessels to be the second most expensive item on the vessel, apart from the cost of the basic hull.
When FPSOs and FSOs began to be used, different types of mooring, tensioning, and brake systems were explored. These types of vessels do not move very often and typically stay at a single location for many years. What was needed but unavailable for these types of vessels was a system where the mooring, which may consist of chain and/or chains and wire rope/strand, is predeployed before the vessel arrives at a location so that when the vessel is on position, the anchor chain need only be picked up on the vessel and tensioned. If this were to be done, it would eliminate the need for large chain lockers which are storage compartments for excess chain, large windlasses, anchor bolsters, and other deck machinery. By elimination of some of this equipment, a significant weight and cost reduction could be achieved, which is very critical for the semisubmersible design. Even in deepwater mono-hulls, weight and space may be a problem. It should be noted that for chain/wire rope systems, some amount of chain is inboard as such to tension the mooring lines.
It is, thus, an objective of the present invention to provide a lightweight, easy-to-install and reliable, simple, fail-safe and economical combination fairlead/tensioner/stopper device that can be built as an integral unit or have the three components separated, depending on the application. It is another objective of the invention to eliminate equipment redundancy so as to save costs, weight and space on the vessel. Thus, for example, the hydraulic power systems that actuate the apparatus can be made portable and moved from one tensioner stopper device to another as the various chains are either tensioned or payed out. These and other advantages of the apparatus will become apparent to those of ordinary skill in the art from a review of the detailed description below.